I'd hold off on a server unless it's a dedicated box you can control, standard MP modes are not what makes ARMA great and if you have a decent PC and fast broadband you can host a COOP game for 6-10 people without much hassle.

The most important thing to have for ARMA is a good Voice Comms server.

I'm inching ever closer to building a new PC, and this game may well push me over the edge into actually doing it. Let's hope it's possible to get through the tutorial without the game shitting itself, which was my experience with Arma II. :-/